A thin film transistor liquid crystal display (TFT-LCD) as a flat panel display device has been used increasingly in the field of high performance display, since it exhibits properties of small volume, low power consumption, zero radiation, relatively low manufacture cost, etc.
The quality of thin film transistors (TFTs) functioning as switch elements is especially important in the TFT-LCD, and a general structure of a TFT may mainly include, as shown in FIG. 1, a source 101, a drain 102, and a gate 103, in which the source 101 and the drain 102 are usually formed using a same layer of metal material, there is further included a semiconductor active layer 11 between regions of source and drain and a gate metal layer, and the semiconductor active layer 11 located over a region of the gate 103 forms a TFT channel 10 (shown by the dashed lines in FIG. 1). In the related art, the material for forming the semiconductor active layer 11 may be amorphous silicon (A-Si). In the TFT having such a structure, mobility is about 0.5 cm2/Vs. However, as the size of a display device is increased gradually, the display device is required to have higher definition and high frequency driving performance. Therefore, the TFT is required to possess high mobility and high performance. In order to enhance the electron mobility of the semiconductor active layer, semiconductor oxide material, e.g., an indium gallium zinc oxide (IGZO), is usually used as the semiconductor active layer of the TFT, which has the electron mobility tens of times higher than that of an amorphous silicon layer.
In the related art, the structure of the TFT using IGZO as the semiconductor active layer 11 may generally adopt a hole type as shown in FIG. 2, in which the source 101 and the drain 102 of the TFT are electrically connected to the IGZO semiconductor active layer 11 through via holes 21 and 22, respectively, or a bar type as shown in FIG. 3, in which the source 101 and the drain 102 of the TFT overlay two sides of the IGZO semiconductor active layer 11. However, the ratio of width to length (W:L) of the TFT channel 10 in the above two structures is relative small, and thus it will take a relative long time for the source and the drain of the TFT to be switched to on-state, which will lower the response speed upon the channel of the TFT turns on down and thereby degrade the display quality of the display device.